1. Field of the Invention
In order to clean a surface for recoating, the technique of using fluid blasting of abrasive particles against a surface has been used. Additionally, blasting techniques have evolved using dry blasting. Dry blasting is comprised of directing a stream of pressurized air containing abrasive media toward the desired surface. Typical air pressures range from 15 to 150 psi.
A typical dry blasting apparatus comprises a dispensing portion, a flexible hose, and a blast nozzle. The dispensing apparatus in which the blast media is contained is a storage tank combined with pressurized air. The flexible hose carries the air/blast media mixture to the blast nozzle and allows the operator to move the blast nozzle relative to the surface to be cleaned. The blast nozzle accelerates the abrasive blast media and directs it into contact with the surface to be treated.
Several factors dictate the efficacy of coating removal. Among those factors are the angle of the nozzle relative to the surface and the distance from the surface. Naturally, effective nozzle positioning will result in optimal speed of coating removal. However, a further factor to consider is the shape of the blast pattern. Nonuniformity of blast media discharge from the nozzle often results in a "hot spot" in the pattern, which is the area of maximum blast media striking the surface being treated at a given moment. A non-uniform blast pattern allows particles to strike already cleaned surfaces which may damage the surfaces and further results in the waste of blast media and compressed air energy. Dispersal of blast media throughout the nozzle is largely governed by the shape of the internal nozzle geometry.
FIGS. 1 and 2 illustrate a blast nozzle in accordance with a prior art example. A cylindrical inlet 4 connects to entry chamber 6 formed by side walls 7 and converging upper and lower walls 8 and 9. The chamber 6 terminates in a throat 10. Following the chamber 6 is a blast chamber 12. The chamber 12 has diverging top and bottom walls 14 and 16, and diverging side walls 18 and 20, to form a rectangular outlet. There is an abrupt change from the cylindrical input to the rectangular entry chamber which causes disrupted flow of particles and a non-uniform output pattern at the output of the blast chamber 12 which results in non-uniform coating removal and the necessity to overlap the stripping path. Overlapping the blasting pattern can result in damaging the surface by blasting previously cleaned surfaces.
Prior art focused on expanding a circular or square hot spot in order to increase efficiency. However, a larger circle or square hot spot is redundant due to the fact that coating removal occurs in a sweeping motion and the real issue regarding efficiency is the width and uniformity of the blast pattern. A long, narrow, rectangular, uniform pattern results in greater efficiency in providing a maximum blast area with evenly distributed particles, which reduces the need to overlap the stripping path, thereby increasing the efficiency and reducing the opportunity to damage the surface.
2. Description of the Prior Art
Different types of nozzles using different shapes are well known in the prior art. These nozzles fail to address the problem described.
U.S. Pat. No. Re. 34,854 to Shank, Jr. shows a fan nozzle with converging, triangular ramps.
U.S. Pat. No. 5,283,990 to Shank, Jr. shows a blast nozzle with a flow straightener intermediate the input and the blast nozzle.
U.S. Pat. No. 5,704,825 to LeCompte shows an inlet portion, an outlet portion and a square venturi orifice connecting the inlet and outlet portions.
In view of the foregoing, it would be highly desirable to provide a long, narrow uniform blast pattern with no hot spot. Such a pattern would represent uniformity of blast media throughout the nozzle and maximum efficiency for surface cleaning.